Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 15 October 2018 doi:10.20944/preprints201810.0280.v1 1 Article 2 Interpretation of sedimentary processes using heavy 3 mineral unconstrained data 4 João Cascalho 1 5 1 Instituto D. Luiz and Departamento de Geologia, Faculdade de Ciências (Universidade de Lisboa). Edifício 6 C6, Campo Grande, 1749-016 Lisboa; [email protected] 7 * Correspondence: [email protected]; Tel.: +351 914238 447 8 9 Abstract: This work describes and interprets the presence of heavy minerals in the western 10 Portuguese continental margin using a set of 78 bottom samples collected from 3 distinct areas of 11 this margin: Porto, Aveiro and Nazaré canyon head areas. The main transparent heavy mineral suite 12 (minerals with frequencies >10%), is composed by amphiboles (hornblende), mica (biotite), 13 andalusite, tourmaline and garnet. A secondary suite (minerals with frequencies between 1 and 14 10%), is composed by pyroxene (enstatite, diopside and augite), staurolite, zircon and apatite. With 15 very low frequency representing less than 1% we found rutile, olivine, kyanite, monazite, epidote, 16 sphene, anatase, sillimanite and brookite. The main primary sources (igneous and metamorphic 17 rocks) explain the presence of these minerals. However, the application of the principal component 18 analysis, with a previous application of the centered log ratio transformation of the heavy mineral 19 data, also stresses for the importance of the grain sorting as a process in controlling the heavy 20 mineral occurrence. The importance of this process is mostly sustained by the distribution pattern 21 of mica and of the most flattened amphibole grains in a way that these particles tend to have a 22 hydraulic affinity to finer grained sediments. 23 Keywords: sand particles; geological sources; grain sorting 24 25 26 27 Graphical abstract. Main heavy minerals: Am-amphibole, Mi-mica, And-andalusite, To-tourmaline, Ga-garnet, 28 Py-pyroxene, St-staurolite, Zi-zircon and Ap-apatite. 29 30 31 © 2018 by the author(s). Distributed under a Creative Commons CC BY license. Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 15 October 2018 doi:10.20944/preprints201810.0280.v1 2 of 20 32 1. Introduction 33 Sedimentology encompasses several areas of expertise including the knowledge of the mineral 34 composition as a tool to understand the sediment provenance [1], from where the heavy minerals 35 emerge as one of the most widely-used techniques [2]. However, to achieve a good estimate of a 36 heavy mineral population hundreds of these mineral particles must be identified and counted in each 37 sample [3,4], which is a high time-consuming routine operation. Therefore, the study of these 38 minerals should be considered a way to increase our knowledge about the sedimentary processes. 39 Many works that use heavy minerals to interpret the sedimentary processes are based on quantified 40 mineral abundances expressed by relative percentages that were computed from mineral counting 41 values [5-7]. However, the interpretation of a heavy mineral suite present in a sedimentary deposit 42 faces several drawbacks. Some of them are derived from natural causes that control the mineral 43 occurrence patterns, such as, the mineralogical composition of the source region or the processes that 44 operate during sedimentary cycles. Even when these drawbacks are known and controlled, we must 45 count on the relationship between sediment grain size and the natural affinity of these minerals to 46 appear more concentrated in certain grain sizes, due to the mineralogical characteristics of the 47 primary source rocks and to the grain sorting process [8-10]. Another difficulty may arise when we 48 apply standard methods of multivariate analysis to a mineral data set expressed in relative 49 abundances ignoring the constant-sum constraint effect [11-12]. 50 Using a set of 78 sea bottom sediment samples collected from the Porto, Aveiro and Nazaré 51 canyon upper heads (Figure 1) we firstly aim to identify the transparent heavy mineral spectrum 52 using the polarized light microscope and, in a complementary way, the electron microprobe analysis. 53 Secondly, we intend to evaluate the relative importance of the main processes in controlling the 54 mineral occurrence by the application of a multivariate statistical method (principal component 55 analysis), avoiding the constant-sum constraint effect by the previous application of the centered log- 56 ratio transformation [12]. Following this method of data analysis, we pretend to distinguish different 57 mineral sources based on the mineralogical composition and on the main grain morphological 58 features. We also intend to evaluate the importance of the grain sorting in the modification of the 59 original mineral signatures of the sources. L A G SPAIN PORTO U T CANYON R O P PORTO AVEIRO AVEIRO CANYON m m 0 0 0 5 2 1 m 0 0 1 N NAZARÉ NYO CA ARÉ NAZ 60 61 Figure 1. Samples location according the 3 sampled areas: Porto, Aveiro and Nazaré canyon head 62 areas. 63 Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 15 October 2018 doi:10.20944/preprints201810.0280.v1 3 of 20 64 2. Geological setting 65 Porto, Aveiro and Nazaré canyons are visible geomorphologic features of the WNW Portuguese 66 continental margin on any bathymetric map (e.g. on the Google Ocean Map). However, while Porto 67 and Aveiro canyons are considered as minor submarine valleys as they weakly indent the shelf [13], 68 the Nazaré canyon is one of the largest canyons of the European Margin (170 km) because it cuts the 69 entire width of the Portuguese Margin, from the Iberia Abyssal plain (at 5000 m depth) until the 70 infralittoral zone off the Nazaré beach [14]. The geological nature of canyon heads surrounding area 71 is also different: Porto canyon is craved in carbonated to detrital rocks highly dolomitized dating 72 from Paleocene; Aveiro canyon is carved in biogenic and detrital limestones rocks dating from 73 Neogenic and Eocenic; Nazaré canyon is craved on Mesozoic rocks [14,15]. 74 The sedimentary cover of the referred canyon head surrounding areas is mainly made by sand 75 with the presence of some other deposits richer in gravel or silt particles [16]. The Porto area reveals 76 a higher grain size variability, ranging between sand and gravel at shallower depths (less than 100m) 77 until fine sediment particles (silt and clay) well represented at middle shelf (Douro muddy deposit) 78 and upper slope, where in some isolated spots of these finer particles can reach up to 70% of the 79 sediment total weight [17]. The Aveiro area reveals a more homogenous sedimentary cover where 80 the sand is the dominant textural type representing always more than 60% of the total sediment. In 81 some small areas, between 100 and 150m depth, the gravel particles can represent up to ⅓ of the total 82 sediment. Finer sediments are only important in some small areas of the upper continental slope with 83 almost 30% of the total sediment weight [16]. The shelf sedimentary cover near to the Nazaré canyon 84 is in some locations dominated by coarse-grained particles (sandy gravel) namely at 40 to 80m depth. 85 At those depths these particles constitute a sedimentary deposit with a geometry sub-parallel to the 86 coast line orientation (paleo littorals). Fine and very fine sands are recorded in the inner shelf north 87 of the canyon and close to its head. Additionally, two important muddy deposits areas are present in 88 the middle shelf north and south of this canyon, at approximately 100 m depth [18,19]. 89 3. Materials and Methods 90 The present work is based on a set of 78 bottom samples, collected from 3 different areas of the 91 northern Portuguese continental margin, that match the Porto (30 samples), Aveiro (26 samples) and 92 Nazaré (22 samples) submarine canyon upper heads (Figure 1). These samples were collected during 93 several cruises during the decades of 1990/99 and 2000/09, using Smith-McIntyre grab on board 94 hydrographical vessels (Almeida Carvalho, NRP D. Carlos I, Andrómeda and Auriga), within the scope 95 of the Portuguese Instituto Hidrográfico program of cartography of the continental shelf sediments 96 (SEPLAT), Sedimentary Dynamics of the Northern Portuguese Continental Shelf project (DISEPLA 97 II), Hotspot Ecossystem Research on the Margins of European Seas project (HERMES) and on the 98 Sedimentary Conduits of the West-Iberian Margin project (DEEPCO). 99 All the samples were first washed using hydrogen peroxide and distilled water to eliminate 100 organic matter and marine salts. Grain-size analysis was done using the classic sieving method for 101 sediments coarser than 0.5mm (at 0.5φ interval) and the laser granulometer (Malvern 2000) for 102 sediments finer than 0.5mm (1φ). The textural statistical parameters (mean and sorting) were 103 computed using the method of moment [20]. Heavy minerals were separated from medium (1φ or 104 0.500mm) to very fine sand (4φ or 0.063mm) using bromoform and then mounted in Canada balsam 105 on glass slides. The required amount of heavy minerals to fill an area of 25x30mm of each slide 106 without grain overlapping was obtained using a micro-splitter. An average of about 380 transparent 107 heavy minerals per sample were counted under the petrographic microscope according to the line 108 counting method [21]. Additionally, to confirm the identities of some specific heavy minerals less 109 easily identified with a standard petrographic microscope (pyroxenes and olivines) some specific 110 grain mounts were made using epoxy resin polished with silicon carbide (sic) and diamond polishing in 111 polishing cloths. These grain mounts were than analyzed by an electron microprobe (JEOL Superprobe 112 733 at Lisbon University).